Between 2 and 4 h after miction the morphology of urinary erythrocytes has changed, clouding the distinction between glomerular and non-glomerular bleeding in pediatric patients. Glomerular bleeding is characterized by microscopically visible alteration in urinary erythrocytes due to glomerular disease. Fixation by Cellfix, a formaldehyde-based fixative, allows the preservation of the morphology for at least 24 h and can be recommended for clinical practice. In our experience, thiomersal was not effective for preservation of the morphology of erythrocytes.